1. Field of the Invention
The present invention relates generally to environmental sensors. More particularly, the present invention relates to micro-electromechanical (“MEMS”) humidity sensors, accelerometers, and resistors.
2. Background of the Invention
Embedding miniature sensors in structures, systems, storage and shipping containers, and other items allows the monitoring of those items to determine health, maintenance needs, lifetime, and other item characteristics. In addition, embedded sensors can be used to perform built in test and evaluation of products. Information from miniature accelerometers, temperature sensors, and humidity sensors can tell a user whether or not the item has been dropped sufficiently to cause damage, experienced temperature extremes beyond specifications, or seen humidity levels beyond those that can be handled. A multifunction sensor suite can be used to collect these environmental parameters, which can then be stored and analyzed by a monitoring system, test stand, or other external device.
Current embedded sensor systems that perform this type of monitoring typically use sets of discrete sensor devices on a printed circuit board to form the sensor suite. The devices are typically individually packaged and interconnected to external electronics using printed circuit board traces. However, this approach limits the miniaturization of the sensor suite. The individual packages take up a lot of space, as do the relatively large circuit traces on a printed circuit board. Monolithic integration of sensors allows size reductions by removing the need for individual packaging for each sensor, as well as utilizing finer electrical interconnects.
Most of the previous work on monolithically integrated sensors has focused on combining two sensor types onto the same chip. Typically, both an accelerometer and a temperature sensor, or both a temperature sensor and humidity sensor, may be monolithically integrated, but not all three. A number of designs and fabrication processes have been employed to perform the integration of two sensors. However, integrating all three sensing functions of accelerometers, temperature sensors, and humidity sensors (or integrating the two sensing functions of accelerometers and humidity sensors) is much more difficult. Integration challenges lie in the sensor design, sensor fabrication, and sensor packaging.
The challenges are due to the sensor devices having substantially different functionality. In particular, the accelerometer requires a suspended inertial mass whereas a humidity sensor requires a material or structure that changes depending on humidity levels. These features often require widely different microfabrication approaches and techniques. Furthermore, a fundamental difference between accelerometers, temperature sensors, and humidity sensors is that the accelerometers and temperature sensors do not need to be exposed to the ambient air or gas. In contrast, humidity sensors need to be exposed to the external environment, as do chemical and biological sensors as well. Integrating exposed sensors and sensors that should not be exposed is difficult from a packaging perspective.
As embedded sensor systems are miniaturized and incorporated into applications, the requirement to use integrated sensor suites becomes more important. It would therefore be desirable to combine accelerometers, temperature sensors, and humidity sensors in a small form factor.